The present invention relates to a gas measuring device, and more particularly to a device for measuring the mass flow or the flow rate of a gas.
It is known to measure gas flow by means of a jet of ions directed perpendicular to the flow. The jet of ions is generated by the corona effect and the deviation in the path of the ions caused by the gas flow is measured. In such known flowmeters, the ions are generated at a source which may be a disc, a point, or a wire and are collected on plane or cylindrical metallic electrodes in such a manner that the ion beam crosses the gas flow substantially perpendicular to the direction of the gas flow. Such flowmeters have the disadvantage of being extremely sensitive to the nature of the electric charge carriers (ions) created in the immediate vicinity of the source, as well as to the variations in the speed of transfer of the ions in going from the source to the collector electrode. Moreover, such known flowmeters have a gas flow passage the cross-section of which is limited to a few centimeters to avoid having to apply too high an ionization voltage, and also to provide sufficient accuracy in the measurements.
Further, in the aforementioned known devices, the accuracy of the measurement is affected by all sources of pollution. If for example the gas is humid, the ions may recombine with molecules of water in the course of their transit, and if the gas whose speed is to be measured comprises different ionizable components, these ionizable components are not subjected to the same deviation as the ions of the jet.
Briefly, a measuring device according to the present invention comprises first and second electrodes positioned in the path of the gas flow, the second electrode being spaced from the first electrode in the flow direction, means for providing an electric potential between the first and second electrodes to enable the first electrode to produce ions for entrainment by the gas flow and for passing through the second electrode, and the first and second electrodes being arranged such that an ion passing through the second electrode leaves the first electrode in a direction substantially perpendicular to the surface of the first electrode or to a plane tangent to the point of emission of the ion at the first electrode surface, and is incident on the second electrode in a direction substantially perpendicular to the surface of the second electrode or to a plane tangent to the point of incidence of the ion at the second electrode surface.
With the arrangement of the present invention, the effect of fluctuations in measurement is substantially unaffected by impurities or pollutants in gas, for example, water particles which may combine with the ions generated. Moreover, measurement is substantially independent of the dimensions of the cross-section of the gas flow passage in the measuring device.
In one preferred form of device embodying the present invention, the device is in the form of a gauge with the gas whose flow characteristics are to be determined traversing a conductive electrode and the ions being created in the gas stream upstream of this conductive electrode. The ion flow is measured which has been able to traverse the electrode under the effect of the gas flow to be determined, this electrode is hereafter called a "transparent" electrode.
In a device embodying the invention, there is obtained a high measure of stability which is not disturbed by fluctuations of the ion source, the determination of the proportion of ions which cross the transparent electrode being independent of the flux of incident ions.
The ion source may be very close to the transparent electrode (network or grating). In practice, their distance is limited only by the dielectric rigidity between the ion source and this transparent grating. Thus, there is substantially no possibility that the ions of the gas to be measured will combine with pollutants. In addition, the voltage to be applied between the ion source and the transparent grating is independent of the cross-section of the gas stream, which enables a low operating voltage to be used in all cases.
In another preferred form of a device embodying the invention, the ion flux which has traversed the transparent grating is measured by means of a collector electrode, also in the form of a grating or network, located downstream of the transparent grating. As a consequence, an average measure of the ions appearing on this collector electrode is obtained and the various fluctuations of the ion source, such as emission noise due to condensation of heavy particles, e.g., dust and smoke, on the source which cause dispersion of the discharge do not disturb the measurement.
If desired, a gauge embodying the invention may have a symmetrical structure with respect to the electric field existing in the inter-electrode space. The inter-electrode space is fixed so as to be constant whatever may be the cross-sectional dimension of the flow passage for the gas through the gauge. With this arrangement, the inter-electrode electric field becomes a constant factor for this type of gauge, once the value of the potential difference applied to the ion source is determined. As a result, such a gauge functions independently of the cross-section of the flow passage for the gas. Thus, only a relatively low voltage source of low cost need be used and the accuracy of measurement, for a given voltage source, is practically the same whatever may be the cross-section of the flow passage for the gas stream. Another advantage, more particularly for large passage sections for the gas stream, is that because of this low voltage source, electrical insulation of the electrodes of the gauge is easy to effect.
Another advantage of such a gauge is that it has an ion source with a very low density of electric charge carriers in its immediate vicinity. In the gauges of the prior art, the density of the ions generated is very high, which results in a disturbance in the inter-electrode electric field as soon as a small variation in this ion density appears. These variations necessarily appear as soon as the gas crossing the gauges contains some impurities. In the case of normal air, e.g., water vapor or smoke is in suspension therein with the result that measurements of the flow rate or flow of such a gas by gauges of the prior art is extremely dependent on such impurities.
In apparatus embodying the present invention, the density of electric charge carriers generated may be extremely small compared with the gauges of the prior art, for example 10-100 times smaller. It is thus evident that variations of this density will then, in terms of absolute value, be smaller in the same ratio, and that the effects on the measurements carried out become practically negligible, and in any case are less than the required accuracy.
Further, this low density feature relates to the aging or reliability of the ion source. The ion sources of gauges of the prior art, owing to the high density of electric charge carriers generated, suffers an intense bombardment by these carriers, resulting in a rapid deterioration of the source material. In contrast, ion sources in a low density gauge embodying the present invention, suffers only very small bombardment. Thus, for an identical material, the aging or deterioration is diminished in a ratio sufficient to permit utilization over a much longer time or more intense utilization.
In order that the present invention may be better understood, various embodiments embodying the invention are illustrated in the accompanying drawings. However, it should be understood that the present invention is not limited solely thereto.